The fraction of Sun-like stars having planets of different sizes, orbiting within 1/4 of the Earth-Sun distance (0.25 AU) of the host star. The graph shows that planets as small as Earth (far left) are relatively common compared to planets 8.0x the size of Earth (similar to Jupiter). For example, 7.8% of Sun-like stars harbor a planet with a size of 1.0 – 1.4 times the size of Earth, orbiting inward of 1/4 the Earth-Sun distance (closer than Mercury’s distance from the Sun). There are increasing numbers of planets from 8x the size of Earth down to 2.8x Earth. Remarkably, the number of planets smaller than 2.8x Earth is approximately constant with planet size, down to the size of our Earth. The gray indicates the planets discovered in this study, and the orange represents the correction applied to account for planets our software would miss statistically, typically about 20%.

As of January 2013, the official Kepler pipeline has detected over 2700 planet candidates. Understanding the distribution of planet sizes, however, remains difficult due to unknown pipeline completeness (i.e. the number of missed planets). Using my own transit search pipeline called “TERRA,” I searched for planets in the quietest 12,000 solar-type stars in the Kepler sample. I detected 129 planet candidates, 37 of which were previously unpublished. Finally, I characterized the number of missed planets through a suite of injection and recovery experiments and produced the de-biased distribution of planet sizes. Remarkably, one in six sunlike stars, hosts a 1-2 Earth-radius planet within 0.25 AU. Also, the rapid rise in planet occurrence toward smaller planet size shown in Howard et al. (2012) stops at ~2.0 Earth-radii, suggesting some aspect of planet formation physics turns off at roughly twice the size of Earth.

Removing systematic errors from Kepler photometry with TERRA. Top: sixty days of raw photometry from Kepler. Bottom: calibrated photometry after removing systematic errors found in a large ensemble of stars. Brightness variations measured in parts per thousand.

Our interest in Earth-size planets has lead Geoff Marcy and me to develop our own transit search pipeline called “TERRA.” When we designed TERRA, we looked at every aspect of a transit search pipeline and asked, “how can we optimize this for small planets?” For example, TERRA identifies systematic noise modes in the time domain shared by a large number of stars, and removes them from each light curve photometry.